The Myth of Snowmelt Thunderstorms

     It happens every year in the spring. Across the mountains of the Western US the TV weather people and even some in the National Weather Service begin to talk about these events and what causes them.

Inevitably as the atmosphere warms up and instability begins to develop over the mountains, cumulus clouds form and some eventually mature into the seasons first thunderstorms. Several years ago, a friend of mine and I began to call these thunderstorms "snowmelt thunderstorms" because of the common misconception that it's the melting mountain snowpack that provides the moisture for these storms to develop. Nothing could be further from the truth.

     Have you ever gone hiking in the mountains in the summer and come across a snow field and noticed how much cooler it is in the general vicinity of that snow? Well it is after all snow and it's cold. As most people know, you typically get thunderstorms when it is hot. Except for perhaps California and the coastal regions of Washington and Oregon thunderstorms in the West occur during the warmer late spring and summer months, not when there is snow on the ground. Snow cover is a heat sink, by that I mean that air over snowpack or even a cold body of water is stable and sinks. To get thunderstorms, you need rising air caused by instability in the atmosphere. If melting snow and that moisture were the cause of these Spring and early Summer thunderstorms then by that reasoning there should be thunderstorms developing every day over Lake Tahoe, and Yellowstone Lake since there is that much more moisture in those regions due to all that snowmelt water. The reality is though that neither of these lakes see thunderstorms develop over them as it is too stable due to the coldness of the water and in effect a mini surface high that sits over them. The only time thunderstorms will develop over these heat sinks is if an outflow boundary or some elevated source of instability aloft triggers a storm. The only other time of year these lakes will see thunderstorms develop over them is in the early fall if an unusually cold airmass moves over the relatively warm lake water creating instability and thus thunderstorms.           

     Thunderstorms develop over the mountains surrounding these areas when there is sufficient instability and surface convergence. The moisture is always there in the atmosphere, it does not need to come from somewhere. Earlier this past Spring, thunderstorms would be forecasted to develop over the Sierra of California but due to the enormous Sierra snowpack this year and the stable atmosphere the snow creates, the storms often developed much further down the West Slopes or east of the East Slopes. Thunderstorms in the Western US DO NOT need moisture from snowmelt or other sources but rather surface convergence(air coming together at ground level), upper divergence(air spreading apart aloft), cooling aloft(decreasing temperatures with altitude) and instability(air that is displaced will rise on it's own). Farther north in my part of the Rockies, you also often need a trigger too such as an upper low or disturbance to enhance the lift as it is simply too stable otherwise even if all the other ingredients are there.

     So the next time you hear on the TV or a meteorologist talking about thunderstorms being caused by moisture from melting snow, send them a snowball and see if the moisture from that melting snowball creates a thunderstorm. =)